My dog’s ears are all over the place. They flop over his head in a haphazard semaphore. It’s hard to imagine how they could serve any practical purpose. My dog’s ears are a dog’s breakfast. Yet sometimes, at unexpected moments, they prick up and turn in rare symmetry towards something: something unknown and unknowable to me, beyond my ken.
It’s summer, the season of sounds: thunderstorms; insects; birdsong; people laughing and talking on the other side of the back fence. It’s the season when after dark all the doors and windows are open and every sound from up and down the street becomes public. On summer nights I find my attention caught without warning by the click of white-striped freetail bats: they fly high and fast and their calls are like a slow telegram, tick, tick, tick, or the wickering of electricity in the cables above the train line as a train approaches from the next station around the bend.
The white-striped freetail bat calls at a frequency of just above 10 kHz. Of all the microbat species—those small insectivorous bats that navigate and find their prey by echolocation—they’re one of the very few that are audible to human hearing. Many more species call at frequencies that are beyond our capacity to hear. A couple of years ago I bought a bat detector so that I could hear them.
A heterodyne bat detector combines a consistent internal frequency with an external frequency, picked up by a microphone, to create a separate third frequency, which is then played through a speaker. In effect, it transposes bat calls. The frequency itself isn’t enough to identify the particular species of bat—though it narrows the field down—but it is enough to let you know if bats are there. And in summer they usually are.
The metabolic cost of flight is high and microbats are voracious feeders. Each bat can eat several thousand insects every night. If you point a bat detector at a spot in the night sky where there are microbats around—say, next to some trees near a watercourse or a bright street light on a warm night—you’ll hear a sound like glitched-out tap dancing, Fred Astaire at an outdoor rave. Different species of microbat call at different frequencies. I’ve picked up ones calling way up at 130 kHz, which is as high as my bat detector goes. Most of the bats I’ve detected in Melbourne, though, are calling in the range of 30–40 kHz or so. Human hearing tops out at about 20 kHz.
Although you have to take into account variables such as the age of the animal and the particular breed, in general dogs can hear up to around 60 kHz. Microbats are well within their range. Some insects, too, call just above the range of human hearing, often in duplicate: switch the bat detector on and turn it up to 25 or 30 kHz and hear the crickets duetting with themselves, the sonic with the ultrasonic. What must a summer night sound like to a dog? A chatter of animal noise, a constant hubbub of calling. If you’ve ever been kept awake by a cricket outside your bedroom window, spare a thought for your dog.
It’s not all animal noise, though. The world we’ve built is a truly noisy one. Fifteen years ago I found myself on top of a hill in the far north of Finland 200 kilometres above the Arctic Circle on a perfectly still day in autumn, and for the first time in my life I experienced a complete absence of external sound. What I was left with was a deafening ringing in my ears, eardrums resonating with the damage of a quarter of a century of constant noise. We’re so used to cars and things powered by electricity that make noise of one kind or another that we forget how recent a phenomenon our noisy world is: we’ve completely transformed our aural landscape in a little over a century.
That landscape doesn’t stop where our hearing does, though. One of my favourite places to go bat detecting is Dights Falls, about a 20-minute walk from my home in Melbourne’s Clifton Hill. At dusk you can just make out against the light pollution of a Melbourne night sky the emergent microbats, flitting about the crowns of the trees or hunting mosquitoes above the swampy ruins of the old mill race next to the falls. But you can’t hear the microbats through a bat detector without also getting an earful of electronic screeching and whining from the lights above the nearby Eastern Freeway.
It sounds like an out-of-tune theremin: a high background hum is the constant noise and it’s punctuated by bursts of crackly squealing. Sometimes the sound seems to be coming from the sky itself, as if you’re tuned into satellites (you’re not, at least not with this model of bat detector: its range is maybe 100 metres). The freeway stretches across the length of the small Dights Falls reserve, and electric lights are all around, but there are definite hotspots in the reserve. My dog, whose ears must be battered by sounds he has no capacity to understand, shows no signs of distress; but in an urban environment full of ultrasonic noise, how would I be able to tell? Is the noise constant, or are there oases of ultrasonic silence? I decided to take my bat detector along with me when I next walked my dog.
There were sounds I was expecting, and duly found: a tangle of wires connecting to an old electricity pole at the end of my street emits a grating whine. When I return from the walk my keys clank and jangle at about 30 kHz, above and in addition to the sound I can hear them making without the detector. Some sounds took me by surprise, though.
Usually I take my dog for a walk along the Merri Creek parkland, but sometimes if I’m pushed for time I’ll do a loop around the smaller soccer field at the end of John Street in the opposite direction: it’s the quickest way to give him some exercise while still making sure that he gets to feel grass beneath his feet. But when I take him down there with the bat detector I’m shocked: the whole field is soaked in that familiar constant electrical whine, grating and screeching. The source is a mobile phone tower that overlooks the whole park. I’ve seen it hundreds of times before but I’ve never really noticed it—surrounded by buildings and trainlines and electrical wires and light poles, it’s just another part of the urban landscape. But aurally, outside our hearing range, it’s unmissable. I decide not to take my dog to that park again.
Not all sounds are so easily avoidable, though. My dog’s collar has two metal loops, a plastic council tag and a metal disc with his name and my phone number engraved on it. When I take him for a walk the metal clip of his lead attaches to one of the loops. This whole assortment of hard surfaces constantly claps together with each step my dog and I take. It sounds like a pleasant metallic tinkling to me, but just above my hearing range it sounds like rusty gears grating together, or like a broken-down car grinding its engine. The noise is awful, and it’s constant. Short of taking my dog’s collar and lead off and hoping that he doesn’t run away, there’s no way to avoid it. I consider trying to wrap the whole assemblage of metal and plastic parts in some kind of bandage to stop them banging against each other.
Not that my dog seems to notice. He’s undemonstrative at the best of times. My vet tells me that they no longer talk about ‘fight or flight’; instead they talk about ‘fight, flight, fidget, or freeze’. My dog’s a freezer. His face is inscrutable. I have no idea if the constant noise that besets him causes him discomfort or distress, or if—like me with the mobile tower in its jumble of visual noise—he just stops noticing it.
We can try to intuit from experience what our pets are thinking, but fundamentally their minds are hidden from us. The same is true of much of our environmental pollution. It’s invisible, or ignored, but it has an effect. Birds in urban environments have been observed to sing differently from their counterparts outside cities: with lower maximum frequencies where there are a lot of hard surfaces, such as glass or concrete, and with higher minimum frequencies in areas with a lot of traffic noise. Both changes represent possible adaptations to the urban environment: lower frequency sounds are more robust and less inclined to be scattered by all those hard surfaces, while higher frequency sounds may coast above the general rumble of the city. Cities also make birds sing more loudly but less coherently, yelling to be heard above the din. Meanwhile, unsurprisingly, the constant noise of human life has been found to have a detrimental effect on the foraging ability of microbats. Some insect species have evolved to ‘jam’ bat echolocation, emitting a sound at a frequency that interferes with the bats’ own calls; human sounds are doing likewise.
I’m writing this on a summer night. I’ve got the window open. My computer squeals and hums and crackles out into the night air, and though I can’t hear those sounds I’ve still got half an ear to the night, as I do every summer, listening for that tick, tick, tick of the white-striped freetail bats flying high and fast and invisible in the night. I’m worried: I’ve realised suddenly that I can’t recall having heard even once what in previous years has been a ubiquitous sound. As the summer continues I become more and more concerned by this absence, but through social media a bat researcher assures me that she hasn’t caught any fewer white-striped freetail bats than in previous years. Oh well, I reason, perhaps I’m just going deaf. It happens: as we get older the upper range of our hearing deteriorates. But then one night while I’m thinking of something else, I suddenly hear it: tick, tick, tick. I punch the air in jubilation like it’s the triumphant end of a movie.
But there’s still a lingering disquiet. They say you don’t know what you’ve got till it’s gone. If that is true then maybe it’s because we don’t notice what we have while it’s here. Scientists warning about the decline of insects often refer to the informal ‘car windscreen test’: how many insects become splattered against the windscreen of your car as you’re driving. How many were there last time you went for a drive? How many do you remember from your childhood? Insecticides soak invisibly into the food chain. Once we kill the insects off, the whole system goes. In 1962 Rachel Carson warned of a silent spring, and the bans on insecticides that her work prompted saved the world from disaster, at least temporarily. But what do we do if the sounds that fall silent are sounds that we can’t even hear?
My dog’s lying just a couple of metres away from me, next to the open window. I switch on my bat detector to try to hear what he might hear, and as he notices my movement he lifts his head, and around his neck the links on his collar clank like a chain. I switch the detector off again, and with a click the world as I perceive it is suddenly more silent.
Harry Saddler is a Melbourne-based writer with a particular interest in interactions between humans and other animals. His book The Eastern Curlew will be published by Affirm Press in 2018.
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